Nmr logging method using superparamagnetic iron oxide nanoparticles

ABSTRACT

The invention is directed to hydrophilic and hydrophobic superparamagnetic nanoparticles and their use as contrast agents for NMR including agents that distinguish oil and water in NMR logging of geological formations containing oil or water. Methods of making these SPIONs are also described.

STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTORS

Aspects of this technology are described by Ali, et al., Synthesis,characterization, and relaxometry studies of hydrophilic and hydrophobicsuperparamagnetic Fe₃O₄ nanoparticles for oil reservoir applications;Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume543, 20 Apr. 2018, Pages 133-143.

STATEMENT OF ACKNOWLEDGEMENT

The support provided by the Deanship of Graduate Studies at King FandUniversity of Petroleum & Minerals (KFUPM), Dhahran, Saudi Arabia, isgratefully acknowledged.

BACKGROUND Field of the Invention

The invention falls within the fields of petroleum sciences andtechnology, material sciences, and nanochemistry.

Description of Related Art

The “background” description provided herein is for the purpose ofgenerally providing a context of the disclosure. Work of the presentlynamed inventors, to the extent it is described in this backgroundsection as well as aspects of the description which may not otherwisequalify as prior art at the time of filing are neither expressly orimpliedly admitted as prior art against the present invention.

One of the most challenging, globally significant, and scientificallydemanding areas in the oil exploration industry is the acquisition ofinformation about and analysis of oil reservoirs; R. B. Bratvold, J. E.Bickel and H. P. Lohne, SPE Reservoir Eval. Eng., 2009, 12, 630-638.Over the last decade, NMR spectroscopy has been used in the petroleumindustry as a complementary tool to conventional methods ofcharacterizing oil formations during the exploration stage. NMR can beused to examine the various types of fluids present in the pores ofreservoir rocks; G. R. Coates, L. Xiao and M. G. Prammer, NMR logging:principles and applications, Gulf Professional Publishing, 1999. Bytuning an NMR probe to a resonant frequency specific regions ofreservoir rocks can be imaged. This NMR-based approach relies on afundamental NMR technique known as spin-spin relaxation or T₂-relaxationin which the transverse component of the magnetization vectorexponentially decays towards its equilibrium position.

There are various factors which can influence the T₂-relaxation time.For instance, the viscosity of a fluid is inversely proportional to T₂.Viscosity is an important parameter during oil exploration andinterrogation of the in-situ molecular dynamics of petroleum fluids.

The concentrations and sizes of superparamagnetic nanoparticles (NPs,SPIONs) also play significant roles as they can alter T₂-relaxation andprovide negative enhancement (i.e., a darker image) in T₂-weightedmagnetic resonance imaging (MM); J. -P. Korb, N. Vorapalawut, B. Nicotand R. G. Bryant, J. Phys. Chem. C, 2015, 119, 24439-24446.

Superparamagnetic iron oxide nanoparticles (SPIONs) have shown theirpotential applications in the fields of magnetic storage—see T. Fried,G. Shemer and G. Markovich, Adv. Mater. (Weinheim, Ger), 2001, 13,1158-1161 and F. Liu, P. Cao, H. Zhang, J. Tian, C. Xiao, C. Shen, J. Liand H. Gao, Adv. Mater. (Weinheim, Ger.), 2005, 17, 1893-1897; incatalysis—see X. J. Wu, R. Jiang, B. Wu, X. M. Su, X. P. Xu and S. J.Ji, Adv. Synth. Catal., 2009, 351, 3150 3156 and M. B. Gawande, P. S.Branco and R. S. Varma, Chem. Soc. Rev., 2013, 42, 3371-3393; inelectrocatalysis—see S. Mitra, P. Poizot, A. Finke and J. M. Tarascon,Adv. Funct. Mater., 2006, 16, 2281-2287 and A. Odagawa, Y. Katoh, Y.Kanzawa, Z. Wei, T. Mikawa, S. Muraoka and T. Takagi, Appl. Phys. Lett.,2007, 91, 133503; biomedicine including for targeted drug delivery—seeJ. H. Maeng, D. -H. Lee, K. H. Jung, Y. -H. Bae, I. -S. Park, S. Jeong,Y. -S. Jeon, C. -K. Shim, W. Kim and J. Kim, Biomaterials, 2010, 31,4995-5006 and J. Kim, J. E. Lee, S. H. Lee, J. H. Yu, J. H. Lee, T. G.Park and T. Hyeon, Adv. Mater. (Weinheim, Ger.), 2008, 20, 478-483;hyperthermia treatments—see D. Shi, H. S. Cho, Y. Chen, H. Xu, H. Gu, J.Lian, W. Wang, G. Liu, C. Huth and L. Wang, Adv. Mater. (Weinheim,Ger.), 2009, 21, 2170-2173 and Q. L. Jiang, S. W. Zheng, R. Y. Hong, S.M. Deng, L. Guo, R. L. Hu, B. Gao, M. Huang, L. F. Cheng, G. H. Liu andY. Q. Wang, Appl. Surf. Sci., 2014, 307, 224-233; and in magneticresonance imaging or MRI as T2-contrast agents—see F. Hu, K. W.MacRenaris, E. A. Waters, E. A. Schultz-Sikma, A. L. Eckermann and T. J.Meade, Chem. Commun. (Cambridge, U. K.), 2010, 46, 73-75 and L. Wang, K.-G. Neoh, E. -T. Kang, B. Shuter and S. -C. Wang, Biomaterials, 2010,31, 3502-3511. Recently, SPIONs have also been investigated asT2-contrast agents for reservoir applications; B. Zhang and H. Daigle,J. Petroleum Sci. and Engin. 162:180-189 (2018). The word “contrast”means the signal differences between adjacent regions, e.g. tissue/boneor tissue/vessel for medical applications and oil/water in terms ofpetroleum or liquid reservoir applications. Typical contrast agents forcomputed tomography and X-rays display contrast enhancements due toelectron-density differences. On the other hand, contrast agents for MMshow contrasting effects based on their interactions with neighboringprotons; H. B. Na, I. C. Song and T. Hyeon, Adv. Mater. (Weinheim,Ger.), 2009, 21, 2133-2148. MRI is based on NMR in which relaxation ofproton spins occurs in the presence of applied magnetic field.Therefore, contrast agents should have the capability to shorten therelaxation time of the neighboring protons; H. B. Na, et al, 2009, id.

It is reported that T₁-based agents provide positive contrastenhancements (e.g., a brighter image) in T₁-weighted MRI, whereas,T₂-based agents deliver negative contrast enhancements (e.g., a darkerimage) in T₂-weighted MM; H. B. Na, et al, 2009, id. and Y. W. Jun, J.H. Lee and J. Cheon, Angew. Chem. Int. Ed., 2008, 47, 5122-5135. T₁ isthe time at which the magnetization reaches 63% of its final value, andthree times T₁ is the time at which 95% polarization is achieved. Fullpolarization of typical reservoir-pore fluids may take several seconds.Large values of T₁ (measured in milliseconds) correspond to weakcoupling between the fluid and its surrounding environment and a slowapproach to magnetic equilibrium, whereas, small T₁ values representstrong coupling and a rapid approach to equilibrium. Different fluids,such as water, oil, and gas, have very different T₁ values. T₁ isdirectly related to pore size and viscosity. The free-induction decay(FID) signal measured in the x-y plane is called T₂—the transverse orspin-spin relaxation. In contrast to T₁, T₂ of hydrocarbons is muchshorter in an inhomogeneous magnetic field. The process of spins losingtheir coherence due to magnetic field inhomogeneity is not a true“relaxation” process and is dependent on the location of the molecule inthe magnet field distribution. Therefore, the FID decay constant isoften referred as T₂* rather than T₂.

The primary objectives in NMR logging are measuring T₁ signal amplitude(as a function of polarization), T₂ signal amplitude and decay, andtheir distributions. The total signal amplitude is proportional to thetotal hydrogen content and is calibrated to give formation porosityindependent of lithology effects. Both relaxation times can beinterpreted for pore-size information and pore-fluid properties,especially viscosity. Depending on the activation used, the computationof a T₁ spectrum requires at least 50% or more, the time needed for thecomputation of a T₂ spectrum. In NMR logging, T₁ measurement initiallyrequired either a stationary mode or very slow logging speeds. With thelatest multi-frequency tools, a technique used for speeding up T₁measurements is to make simultaneous measurements of the individualsteps observed during a T₁ recovery experiment in adjacent volumes; atleast two such volumes are required. This technique enables T₁acquisition in less time, thereby permitting faster logging speeds; seehttps://_petrowiki.org/Nuclear_magnetic_resonance_(NMR)_logging (lastaccessed Aug. 20, 2018, incorporated by reference).

SPIONs are promising candidates to give T₂ contrast properties (darkerimage) in T₂-weighted MRI. The inventors measured T₂ relaxation signalsand r₂ relaxometry properties. The functional parameters of size, shapeuniformity, colloidal stability and superparamagnetic characteristicsdistinguish the SPIONs of the invention from other magnetic particles.For example, the SPIONs synthesized by the inventors have betterstability and efficiency than the commercial contrast agents as shownherein.

The efficiency of contrast agents is usually expressed in terms oflongitudinal (r₁) and/or transversal (r₂) relaxivity. The higher valuesof r₁ and r₂ are related to the T₁-positive and T₂-negative contrastenhancements, respectively; Z. Li, P. W. Yi, Q. Sun, H. Lei, H. Li Zhao,Z. H. Zhu, S. C. Smith, M. B. Lan and G. Q. M. Lu, Adv. Funct. Mater.,2012, 22, 2387-2393.

Among various forms of iron oxides, magnetite (Fe₃O₄) NPs exhibit avariety of potential applications owing to a high Curie temperature(T_(C) ^(bulk)˜850 K at T_(v)≈125 K) and highest saturationmagnetization of (M_(S) ^(bulk)˜92 emu/g) among the oxides of iron; V.N. Nikiforov, Y. A.

Koksharov, S. N. Polyakov, A. P. Malakho, A. V. Volkov, M. A. Moskvina,G. B. Khomutov and V. Y. Irkhin, J. Alloys Compd., 2013, 569, 58-61.Moreover, the magnetic behavior of magnetite NPs is related to theirsize and it is well-established that magnetite NPs show transition frommulti-domain to single-domain magnetic structures as the size decreasesbelow 90 nm.

Upon further reduction in the size to below 30 nm, these NPs can revealsuperparamagnetic behavior at room temperature; K. M. Krishnan, IEEETrans. Magn., 2010, 46, 2523-2558. If the size of NPs≥30 nm, thencoercive forces may dominate and can cause aggregation in the presenceof strong external magnetic field:, Y. V. Kolen'ko, M. Bañobre-López, C.Rodriguez-Abreu, E. Carbó-Argibay, A. Sailsman, Y. Piñeiro-Redondo, M.Cerqueira, D, Y. Petrovykh, K. Kovnir and O. I. Lebedev, J. Phys. Chem.C, 2014, 118, 8691-8701. However, the synthesis of stable anddispersible SPIONs having ultra-small size is very challenging and hencedistinct research efforts are required to prepare SPIONs of desirableproperties for NRM and MRI applications.

Various synthetic methods, e.g., coprecipitation (G. Wang, X. Zhang, A.Skallberg, Y. Lira, Z. Hu, X. Mei and K. Uvdal, Nanoscale, 2014, 6,2953-2963), solvothermal (V. Patsula, L. Kosinová, M. Lovrić, L.Ferhatovic Hamzić, M. Rabyk, R. Konefal, A, Paruzel, M, Šlouf, V.Helynek and S. k. Gajović, ACS Appl. Mater. Interfaces, 2016, 8,7238-7247 and X. Lians, Ci. Ji, L. Zhang, Y. Yang and X. Liu, GlassPhys. Chem., 2011, 37, 459-465), hydrothermal (Y. V. Kolen'ko, et al,,2014, id and X. -D. Liu, H. Chen, S. -S. Liu, L. -Q. Ye and Y. -P. Li,Master. Res. Bull., 2015 62, 217-221,, polyol (M. Abbas, B.Parvatheeswara Rao, S. M. Naga, M Takahashi and C Kim, Ceram. Int.,2013, 39, 7605-7611 and M. Abbas, B. P. Rao., M. N. Islam, S. Naga. MTakahashi and C. Kim, Ceram. Int, 2014, 40, 1379-1385), thermaldecomposition Z. Xu, C. Shen, Y. Hou, H. Gao and S. Sun, Chem. Mater.,2009, 21, 1778-1780) and thermolysis (J. Cha, P. Cui and J. -K. Lee, J.Mater. Chem., 2010, 20, 5533-5537) have been adopted for the synthesisof hydrophilic and hydrophobic SPIONs.

Magnetite NPs coated with a silica shell (Fe₃O₄@SiO₂) have been preparedvia a modified Stober process using Fe₃O₄ seeds (C. Hui, C. Shen, J.Tian, L. Bao, H. Ding, C. Li, Y. Tian, X. Shi and H. -J. Gao, Nanoscale,2011, 3, 701-705). It was observed that the silica shell thickness couldbe controlled by tuning various experimental parameters such as theconcentration of seeds, the ratio of tetraethyl orthosilicate(TEOS)/Fe₃O₄ and reaction termination time. The synthesized NPsexhibited superparamagnetic behavior at room temperature. Similarly,Fe₃O₄@SiO₂ NPs having hydrophilic characteristics were synthesized bythermolysis using poly (vinyl pyrrolidone) (PVP), and TEOS in thepresence of NH₄OH catalyst; J. Cha, et al., 2010, id. However, thesesynthetic processes involved tedious surface treatments and multi-stepprocedures including hydrolysis and condensation reaction of TEOS.

Abbas et al., 2013, id. reported a single-step polyol method for thesynthesis of hydrophilic SPIONs using polyethylene glycol (PEG) as asolvent and a stabilizing agent. The pH of the solution was adjusted to˜10 by the addition of NaOH and reaction was carried out at a relativelyhigher temperature (300° C.). Abbas, et al. 2014, id. reported amodified polyol method for the encapsulation of a silica shell on thesurface of magnetite NPs to induce hydrophilicity under similar reactionconditions. U.S. Pat. Nos. 8,828,357, 8,383,085 and U.S. PatentPublication U.S. 2017/0266670 also describe multistep methods for makinghydrophobic FeO or Fe₃O₄ nanoparticles.

Using another appoach, Xu and his co-workers reported a single-stepthermal decomposition method for synthesis of monodisperse hydrophobicSPIONs in which the size of NPs was tuned by varying the volumetricratio between oleylamine (OLA) and benzyl ether; Z. Xu, et al., 2009,id. However, the thermal decomposition of iron acetyl acetonateFe(acac)₃ was carried out in the presence of highly flammable benzylether at a high temperature (300° C.) making it quite hazardous.

In view of the limitations of conventional methods, the inventors soughtto provide a safer, more convenient way to produce SPIONS havingsuperior properties as contrast agents and which had the hydrophilic orhydrophobic characteristics and stability necessary for interrogatingpetroleum and other fluid reservoirs.

BRIEF SUMMARY OF THE INVENTION

As disclosed herein, the inventors now provide a single-step, facilesolvothermal method for the synthesis of SPIONs having the hydrophilicor hydrophobic characteristics, size, and stability necessary forinterrogating geological formations and petroleum and other fluidreservoirs. In these methods solvents such as polyethylene glycol oroleylamine act as reducing, stabilizing and capping agents and theinventors show that PEG-400 (Mn 380-420 g/mol) and OLA, respectively,can be selectively employed to provide hydrophilic and hydrophobicfunctionalities on the surface of SPIONs. The SPIONs produced by amethod according to the invention were characterized by severalphysicochemical techniques and compared to uncoated-Fe₃O₄ NPs which wereprepared by a coprecipitation method using NH₄OH as precipitating agent.The stability of functionalized SPIONs made according to the inventionwas monitored in representative environments, such as artificial seawater (ASW) or model oil (cyclohexane-hexadecane 1:1) and the quenchingof T₂-relaxation signals produced using various concentrations of SPIONswas determined. Both hydrophilic- and hydrophobic-functionalized SPIONswere found to exhibit excellent relaxivity properties compatible withthe use as T₂-contrast agents for oil reservoir applications.

Embodiments of this technology include but are not limited to thosedisclosed below. One aspect of the invention is directed to a method formaking SPIONS that includes (i) for hydrophilic-Fe₃O₄ SPIONS: mixing aniron-containing precursor with a hydrophilic ligand or capping agent toform a homogeneous suspension; heating the homogenous suspension to atemperature sufficient to bind the hydrophilic ligand to theiron-containing precursor, cooling the resulting homogenous suspension,adding a solvent to the cooled homogeneous suspension to precipitatehydrophilic-Fe₃O₄ SPIONs into a slurry, separating the precipitatedhydrophilic-Fe₃O₄ SPIONs from the slurry, and washing thehydrophilic-Fe₃O₄ SPIONs to remove unbound hydrophilic ligands orcapping agent, thereby making hydrophilic hydrophilic-Fe₃O₄ SPIONS; or(ii) for hydrophobic-Fe₃O₄ SPIONS: mixing an iron-containing precursorwith a hydrophobic ligand or capping agent to form a homogeneoussuspension; heating the homogenous suspension to a temperaturesufficient to bind the hydrophobic ligand or capping agent with theiron-containing precursor, cooling the homogenous suspension, adding asolvent to precipitate hydrophobic-Fe₃O₄ SPIONS into a slurry,separating hydrophobic-Fe₃O₄ SPIONS from the slurry, and washing thehydrophobic-Fe₃O₄ SPIONS to remove unbound hydrophobic ligand or cappingagent, thereby making hydrophobic-Fe₃O₄ SPIONS. In some embodiments,iron(III) ethylhexanoate can be substituted from iron(III)acetylacetonate; polyethylene glycol-based surfactants may be usedinstead of PEG; and oleic acid may be replaced with oleylamine.

Variables for the solvothermal process include: (i) nature of solvent,(ii) nature of precursors and (iii) synthesis temperature. These factorsmay affect the particle size and shape homogeneity, which consequentlyresults in changes to T₁ and T₂ values. The inventors observed thatthese variables as they select for smaller particle size and bettershape homogeneity gave better relaxometry properties and vice versa.

In some embodiments, this method will be a single-step solvothermalmethod wherein the hydrophilic and hydrophobic ligands or capping agentsparticipate in reducing, stabilizing and capping of the SPIONs producedwhich can then be recovered, washed and further processed, for example,dried.

In some embodiments, the heating is performed in an autoclave or otherclosed controlled environment at a pressure ranging from 5, 10, 20, 30,40, 50, 60, 70, 80, 90, 100 or >100 psi.

When this method is used to produce hydrophilic-Fe₃O₄ SPIONs, theiron-containing precursor may be selected to be iron(III)acetylacetonate, the hydrophilic ligand or capping agent selected to bepolyethylene glycol (PEG), and the heating selected to be at atemperature that is less than the standard boiling point for saidhydrophilic ligand or capping agent; the solvent may be selected to beat least one alcohol, ketone, ether or other organic solvent. Thehydrophilic-SPIONs may be washed in at least one alcohol, ketone, etheror other organic solvent.

When this method is used to produce hydrophobic-Fe₃O₄ SPIONs, theiron-containing precursor may be selected to be iron(III)acetylacetonate, the hydrophilic ligand or capping agent selected to beoleylamine (OLA) or another unsaturated fatty amine, and the heatingselected to be at a temperature that is less than the standard boilingpoint for said hydrophobic ligand or capping agent; the solvent may beselected to be at least one alcohol, ketone, ether or other organicsolvent. The hydrophobic-SPIONs may be washed in at least one alcohol,ketone, ether or other organic solvent.

In further embodiments of the invention, the hydrophilic-Fe₃O₄ SPIONsare made from an iron-containing precursor that is iron(III)acetylacetonate, the hydrophilic ligand or capping agent is polyethyleneglycol 400 (PEG-400; Mn 380-420 g/mol), the heating is at a pressure of15 to 45 psi and at a temperature ranging from 160, 165, 170, 175, 180,190, 195-200° C., preferably from 175 to 185° C. for at least 12, 18,24, 30, to 36 hours, the cooling the mixture is to <15, 20, 25, 30, 35or >35° C. (or any intermediate temperature), the hydrophilic-SPIONs arewashed in a mixture of absolute ethanol and diethylether.

In other further embodiments of this method where hydrophobic-Fe₃O₄SPIONs are made, the iron-containing precursor is selected to beiron(III) acetylacetonate, the hydrophobic ligand or capping agent isoleylamine (OLA), the heating is at a pressure of 40 to 80 psi and attemperature ranging from 260, 265, 270, 275, 280, 285, 290 to 300° C.,preferably from 275 to 285° C. for at least 12, 18, 24, 30, to 36 hours,the cooling the mixture is to <15, 20, 25, 30, 35 or >35° C. (or anyintermediate temperature), the hydrophobic-SPIONs are washed in amixture of absolute ethanol and diethylether.

Advantageously, a solvothermal method may be performed in a sealedautoclave that is safer and more controlled than performing thermaldecomposition of organic precursors in open atmosphere at hightemperature. In the synthesis of both PEG-SPIONs and OLA-SPIONs highlystable organic solvents such as PEG-400 and oleylamine are used which,respectively have boiling points of ˜240° C., and ˜364° C.

In some embodiments, during the synthesis of PEG-coated Fe₃O₄ NPs(SPIONS) by a single step solvothermal method, the autoclave pressuremay range from 20, 25, 30, 35 or 40 psi and autoclave temperature rangefrom 160, 170, 180, 190 or 200° C.; preferably, the approximate value ofpressure inside the autoclave will be ˜30 psi at 180° C. These rangesinclude all intermediate subranges and values.

In other embodiments, during a single-step solvothermal synthesis ofoleylamine coated Fe₃O₄ NPs in an autoclave, the approximate pressurevalue inside the autoclave will be about 50, 55, 60, 65 or 70 psi,preferably about 60 psi, at a temperature ranging from 260, 270, 280,290 or 300° C., preferably at a temperature of about 280° C. Theseranges include all intermediate subranges and values.

Typically, the solvothermal methods disclosed herein where temperatureand pressure are controlled will safely synthesize and yieldhighly-dispersed, monocrystalline, size- and shape-controlled s SPIONsin an environment.

Another embodiment of the invention involves superparamagneticnanoparticles (“SPIONs”) that include (i) a core that contains magnetiteand hydrophilic functional groups bonded to a surface of the core,wherein the hydrophilic functional groups include polyethylene glycolmoieties; wherein the hydrophilic superparamagnetic nanoparticles have aspheroidal or spherical morphology with a TEM diameter of about 8, 9,10, 11, 12, 13, 14, 15, or 16 nm and a transversal relaxivity (r₂) forPEG-Fe₃O₄ of about 61, 62, 63, 64, 65, 66, 66.7, 67, 68, 69, 70, 71, or72, mM⁻¹s⁻¹, advantageously about 66.7, and exhibit thermal stability ininert conditions with substantially no phase transformation between 200,250, 300, 350, 400, 450, or 500° C.; or that include (ii) a core thatcontains magnetite; and hydrophobic functional groups bonded to asurface of the core, wherein the hydrophobic functional groups includeoleylamine moieties, wherein the hydrophobic superparamagneticnanoparticles have a spheroidal or spherical morphology with a diameterof about 10, 11, 12, 13, 14, 15, 16, 17, or 18 nm and have a transversalrelaxivity (r₂) for OLA-Fe₃O₄ of about 45, 46, 47, 48, 49, 50, 51, 52,53, or mM⁻¹s⁻¹, advantageously about 49.0, and which exhibit thermalstability with substantially no phase transformation between 200, 250,300, 350, 400, 450, or 500° C.

The hydrophilic SPIONs of the invention, such as PEG-SPIONs, may exhibita phase composition of magnetite, a crystallite size ranging from about10, 10.5, 11, 11.5, 12, 12.5, 13, 13.3, 13.5, 14, 14.5, 15, 15.5 toabout 16, preferably about 13 nm; and/or a unit cell volume ranging from560, 565, 570, 580, 588, 590, or 600 Å³preferably about 588 Å³. Inpreferred embodiments, the hydrophilic SPION will not contain reactivesurface groups such as carboxylic acid or amine groups.

The hydrophobic SPIONs of the invention, such as OLA-SPIONs, may exhibita phase composition of magnetite, a crystallite size ranging from about11, 11.5, 12, 12.5, 13, 13.5, 14, 14.1, 14.5, 15, 15.5, 16, 16.5 to 17,preferably about 14 nm; and/or a unit cell volume ranging from 560, 565,570, 575, 580, 585, 589, 590, 595, or 600 Å³preferably about 589 Å³. Inpreferred embodiments, the hydrophobic SPION will not contain reactivesurface groups such as carboxylic acid or amine groups.

In some embodiments, the surface of the magnetite core of the SPIONS isbonded to polyethylene glycol moieties or other hydrophilic groups, inother embodiments the surface is bonded to OLA moieties or otherhydrophobic groups. In one embodiment, the surface of the magnetite coreis bonded to PEG moieties; in other embodiment, the surface of themagnetite core is bonded to OLA moieties. The SPIONS disclosed hereinmay be made by a solvothermal method described below.

Transversal relaxivity of PEG-SPIONs and OLA-SPIONs was determined inartificial sweater (ASW) and model oil (cyclohexane: hexadecane, 1:1),respectively, at ambient temperature and pressure.

PEG-SPIONs of the invention are equally efficient and more stable ascompared to SHU-555C (r₂=69 mM⁻¹s⁻¹).

PEG-SPIONs and OLA-SPIONs of the invention are more stable, efficientand less expensive as compared to Gd-DTPA (r₂=5.3 mM⁻¹s⁻¹).

In another embodiment, the invention is directed to drilling mud ordrilling fluid that includes the hydrophilic or hydrophobic SPIONsdisclosed herein such as PEG-SPION or OLA-SPIONs. As described hereinthere are three main categories of drilling fluids which are water-basedmuds, which can be dispersed and non-dispersed, non-aqueous muds, oftencalled oil-based muds, and gaseous drilling fluids, in which air or awide range of other gases can be used. Drilling fluids include water,seawater, brines as well as compositions containing additional or otheringredients such as air or other gases, surfactants, bentonite and otherclays as well various kinds of muds. Other components of drilling fluidare described by https://_en.wikipedia.org/wiki/Drilling_fluid (lastaccessed Aug. 15, 2018, incorporated by reference). The hydrophilicSPIONs of the invention may advantageously be incorporated into drillingmuds or reservoirs containing aqueous ingredients or materials; thehydrophobic SPIONs of the invention may advantageously incorporated intohydrophobic drilling muds or reservoirs containing oil or otherhydrophobic materials. PEG-SPIONs (hydrophilic) are suitable forwater-based drilling fluids (WDF) prepared with barite, ilmenite, and/ormanganese tetroxide. While OLA-SPIONs (hydrophobic) are suitable foroil-based drilling fluids (ODF) having weighting agents such as barite,ilmenite, and manganese tetroxide.

Another embodiment of the invention is a geological formation or fluidreservoir containing or doped with the hydrophilic or hydrophobic SPIONsdisclosed herein. The geological formation may contain porous rockshaving a variety of pore sizes, impermeable rock, water, brine, oil orgas or mixtures of these which may be contained in porous rocks or nextto between layers of impermeable rock. The geological formation may beunder dry land or underwater, such as beneath a lake, sea or ocean.SPIONs may be incorporated into a geological formation by one or moreinjectors, through one or more boreholes, or during drilling, forexample, in combination with water, brine, injection surfactantmixtures, or drilling fluids or drilling muds.

In another embodiment, the invention is directed to a nuclear magneticresonance (NMR) or magnetic resonance imaging (MRI) procedure involvingcontacting a material such as rock or a geological formation, or a body,tissue or vessels, with a contrast agent comprising the SPIONs disclosedherein. Methods for biomedical or clinical use of MRI contrast agentsare known in the art and incorporated by reference tohttps://_en.wikipedia.org/wiki/MRI_contrast agent (last accessed Aug.16, 2018) as are methods for using contrast agents or magnetic tags inpetroleum exploration or logging.

Another embodiment of the invention is directed to a method for NMRlogging of a geological formation that includes injecting or otherwiseincorporating the hydrophilic and/or hydrophobic SPIONs into thegeological formation for example through a borehole or during thedrilling of a borehole, generating a magnetic field and detecting amagnetic signal. This method may be performed with SPIONs that arehydrophilic such as PEG-SPIONs or with SPIONs that are hydrophobic suchas OLA-SPIONs. In some embodiments both hydrophilic and hydrophobicSPIONs may be used, for example, by targeting portions of the geologicalformation containing water with hydrophilic SPIONs and those with oil orother non-aqueous components with hydrophobic SPIONs. Some embodimentsof this method involve logging-while-drilling (LWD) logging and/orwireline logging. SPIONs may be introduced into a geological formationthrough a bore hole or concurrently with drilling. SPIONs may beinjected into oil and/or water or brine present in a fluid reservoir inthe geological formation or into or around porous or non-porous rock. Insome embodiments, these methods will detect or measure a magnetic signalis a T1 signal and/or a T2 signal. PEG- and OLA SPIONs can be useful toenhance T₁ contrast properties.

In another embodiment the SPIONs of the invention are employed in amethod for tracking, tracing, or quantifying oil and/or water in ageological formation or fluid reservoir by injecting them into thegeological formation or fluid reservoir and detecting a concentration ofSPIONs in oil and/or water recovered from the geological formation orfluid reservoir. In some embodiments, the same or different kinds ofSPIONs may be introduced into a geological formation through differentbore holes; see C. Huh, N. Nizamidin, G. a. Pope, T. E. Milner, and B.Wang, “Hydrophobic paramagnetic nanoparticles as intelligent crude oiltracers,” pp. 1-10, 2014. WO 2014123672 A1, incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A. Functionality and stability test of PEG-Fe₃O₄, OLA-Fe₃O₄, andFe₃O₄ SPIONs in oil-DI water (oil-deionized water). Stability ofPEG-Fe₃O₄ (samples a, d), OLA-Fe₃O₄ (samples b, e), and Fe₃O₄ (samplesc, f) in the oil-DI water phase.

FIG. 1B. Functionality and stability test of PEG-Fe₃O₄, OLA-Fe₃O₄, andFe₃O₄ SPIONs in oil-seawater phase. Stability of PEG-Fe₃O₄ (samples a,d), OLA-Fe₃O₄ (samples b, e), and Fe₃O₄ (samples c, f) in oil-seawaterphases.

FIG. 2 shows diffractograms of three types of magnetic NPs synthesizedusing PEG, OLA, and NH₄OH (top to bottom, respectively).

FIG. 3A surveys PEG-Fe₃O₄ (top), OLA-Fe₃O₄ (middle), and Fe₃O₄ (bottom)spectra showing Fe2p, N1s, O1s, and C1s features.

FIG. 3B. XPS profile of SPIONs, Fe2p. This figure compares XPS profilesof PEG-Fe₃O₄ (top), OLA-Fe₃O₄ (middle), and Fe₃O₄ (bottom) freshlysynthesized powder samples.

FIG. 3C. Observed (black line) and fitted (blue line) XPS spectra of N1sfor OLA-Fe₃O₄. This figure depicts a symmetric peak with low intensitydetected at 399.7 eV in the N1s spectrum attributed to —NH₂ group ofOLA.

FIG. 3D. Deconvoluted high-resolution XPS spectra of the O1s componentfor PEG-Fe₃O_(4;) observed (black line), fitted (pink line), Fe—O (redline), —OH (blue line), and C—O (green line) spectra.

FIG. 3E. Deconvoluted high-resolution XPS spectra of the O1s componentfor OLA-Fe₃O_(4;) observed (black line), fitted (cyan line), Fe—O (redline), and —OH (green line) spectra.

FIG. 3F. Deconvoluted high-resolution XPS spectra of the O1s componentfor Fe₃O₄; observed (black line), fitted (cyan line), Fe—O (red line),and —OH (green line) spectra.

FIG. 4 . Comparative DSC curves of Fe₃O₄, OLA-Fe₃O₄, PEG-Fe₃O₄ SPIONsalong with oleylamine (OLA) and polyethylene glycol (PEG). This figureshows the thermal behavior of uncoated and coated SPIONs in thetemperature range of 20-500 ° C.

FIG. 5A. High-resolution FESEM image of PEG-Fe₃O₄.

FIG. 5B. High-resolution FESEM image of OLA-Fe₃O₄.

FIG. 5C. High-resolution FESEM image of Fe₃O₄ NPs.

FIGS. 6A-6F. High-resolution TEM images (FIGS. 6A-6C) and SAED patterns(FIGS. 6D-6F) of PEG-Fe₃O₄, OLA-Fe₃O₄, and Fe₃O₄ SPIONs, respectively.

FIGS. 7A-7C show T₂-relaxation curves observed for various Feconcentrations of PEG-Fe₃O₄ SPIONs (in artificial seawater), OLA-Fe₃O₄SPIONs (in oil), and Fe₃O₄ SPIONs (in artificial seawater),respectively. In FIGS. 7A-7C, the graphs from top to bottom follow theorder in the boxed legend with 0.012 mM sample being shown by the topgraph and 0.48 mM sample by the lowest.

FIG. 7D shows an inverse of relaxation time (1/T₂) versus Feconcentration of as-synthesized SPIONs. Top line=PEG-Fe₃O₄ (r₂=66.7mMs⁻¹); middle line=OLA-Fe₃O₄ (r₂=49.0 mMs⁻¹); and bottom line (r₂=32.2mMs⁻¹)=Fe₃O₄.

FIG. 8A. Low-resolution FESEM image of PEG-Fe₃O₄ NPs (SPIONs)

FIG. 8B. Low-resolution FESEM image of OLA-Fe₃O₄ NPs (SPIONs).

FIG. 8C. Low-resolution FESEM image of Fe₃O₄ NPs (SPIONs).

FIG. 9A. Low-resolution TEM image of PEG-Fe₃O₄ NPs (SPIONs).

FIG. 9B. Low-resolution TEM image of OLA-Fe₃O₄ NPs (SPIONs).

FIG. 9C. Low-resolution TEM image of Fe₃O₄ NPs (SPIONs).

FIG. 10 . T₂-relaxation curves and relaxation time measurement of DIwater, artificial sea water (ASW), cyclohexane, hexadecane, and modeloil.

DETAILED DESCRIPTION OF THE INVENTION

The inventors disclose herein a safe and simple single-step solvothermalmethod for the synthesis of highly-stable hydrophilic and hydrophobicsuperparamagnetic iron oxide nanoparticles (SPIONs) as T₂-contrastagents. These SPIONs have contrast properties and stabilities thatpermit their employment in petroleum exploration and monitoring, such asin interrogation of oil reservoirs.

The invention provides T₂ contrast agents with several superiorproperties including (i) quenching of T₂-relaxation signals with selectSPIONs concentrations, (ii) excellent relaxivity properties due toultra-small SPION size, and (iii) long-term stability including thermalstability in different media. These properties permit use of the SPIONsof the invention in harsh conditions often found in petroleumreservoirs.

A petroleum reservoir or oil and gas reservoir is a subsurface pool ofhydrocarbons contained in porous or fractured rock formations and may bebroadly classified as a conventional or unconventional reservoir. In aconventional reservoir, the naturally occurring hydrocarbons, such ascrude oil or natural gas, are trapped by overlying rock formations withlower permeability. While in an unconventional reservoir the rocks havehigh porosity and low permeability which keeps the hydrocarbons trappedin place.

The SPIONs made by a method according to the invention may be used ascontrast agents for magnetic (e.g., NMR) characterization orinterrogation of a conventional or unconventional petroleum, gas orother liquid reservoir (or as contrast agents in magnetic imaging suchas MRI).

In some embodiments, the SPION-based methods for characterization orinterrogation of a petroleum reservoir may be used in conjunction with aseismic survey, appraisal well, and computer modelling of a reservoir.These combined techniques may be used to assess the size, volume oruniformity of a reservoir, compartmentalization of a reservoir, thelocation of oil-water contact in a reservoir, height of oil-bearingsands, rock porosity, percentage of rock containing fluids or percentageof solid rock, estimate the amount of petroleum or gas or other fluid ina reservoir and recovery factor (proportion of recoverable oil or gas).Data obtained by use of the SPIONs of the invention and conventionaltechniques may be used to help build a computer model of a reservoir.

A SPION is a superparamagnetic iron oxide nanoparticle. As demonstratedherein the surface of a SPION may be functionalized to make it morehydrophilic or hydrophobic, enhance its stability under particularconditions, such as in the presence of water, sea water, mixtures ofwater or sea water and oil, or in oil or other petroleum materials, oraffect its ability to be magnetically detected or imaged (e.g., by NMRor MRI). It may be functionalized to reduce its ability tonon-specifically bind to a substrate and to improve its ability toassociate with a particular target substrate.

A SPION may be made of magnetite Fe₃O₄ and/or its oxidized formmaghemite or γ-Fe₂O₃. In some embodiments, a SPION of the invention willcontain Fe₂O₃ or a mixture such as NiFe₂O₄, CuFe₂O₄, MnFe₂O₄ or CoFe₂O₄.In some embodiments a SPION, exclusive of functionalization, willconsist of Fe₂O₃ and exclude other metallic components such as thosedescribed above or metals such as gadolinium. In many embodiments, theSPIONs of the invention will exhibit substantial thermal stability withno phase transformation between 200, 250, 300, 350, 400, 450 and 500° C.

In NMR, T1 relaxation is the process by which the net magnetization (M)grows/returns to its initial maximum value (Mo) parallel to B_(o).Synonyms for Tlrelaxation include longitudinal relaxation, thermalrelaxation and spin-lattice relaxation.

T2 relaxation is the process by which the transverse components ofmagnetization (Mxy) decay or dephase. In medical MRI (NMR-basedimaging), T1 images are used to highlight fat tissue, while T₂ imageshighlight fat and water.

The relaxation rates (r₂ or 1/T2) for PEG-SPIONs of the invention mayrange between 60, 61, 62, 63, 64, 65, 66, 66.7, 67, 68, 69, 70, 71,72-73 mM⁻¹s⁻¹, preferably about 66.7 ; and for OLA-SPIONS from about 44,45, 46, 47, 48,49.0, 50, 51, 52, 53-54 mM⁻¹s⁻¹, preferably about 49.0.

SPIONs are usually T₂₋based contrast agents and T₂ contrast is oneaspect of the invention. Hydrophilic-SPIONs help to alter T₂ signalproduced from water, while hydrophobic SPIONs change T₂ signal producefrom oil. T₂ signals coming from brine saturated with hydrophilic SPIONsare different than T₂ signals of brine alone. Similarly, T₂ signalsreceived from oil having with hydrophobic SPIONs is different than T₂signals of oil alone. T₁-based contrast agents provide positive contrastenhancement (i.e., brighter image) which also help to distinguish thewater and oil phases in the porous rock. T₁-based contrast agentsprovide positive contrast enhancement (i.e., brighter image) which alsohelp to distinguish the water and oil phases in the porous rock.

Moreover, there is no significant advantage to use the combination ofhydrophilic and hydrophobic SPIONs. T₂ signals coming from brinesaturated with hydrophilic SPIONs is different than T₂ signal of brinealone. Similarly, T₂ signal receive from oil having with hydrophobicSPIONs is different than T₂ signal of oil alone.

Advantageously, PEG-SPIONs (hydrophilic) can exhibit colloidal stabilityin water as well as in seawater. However, OLA-SPIONs (hydrophobic) canprovide the colloidal stability in model oil.

The average diameter of a SPIONs as disclosed herein may range from nm,2 to 30 nm such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,17, 18, 19, 20,21, 22, 23, 24, 25, 26, 27, 28, 29 to 30 nm, andadvantageously from 9, 10, 11, 12, 13, 14, or 15 nm. When the size of aSPION is greater than 30 nm, coercive forces may dominate and can causeaggregation in the presence of strong external magnetic field.

The preferred average diameter of particles is ≤20 nm in order toachieve better relaxometry properties.

Polyethylene glycols useful in a method according to the invention maybe selected from liquid PEGs, such as those having number averagemolecular weights (Mn g/mol) of 200, 300, 400, 500, or 600 g/mole aswell as all intermediate values within this range. PEGs may act asreducing and/or stabilizing agents that confer hydrophilic properties ona SPION. Advantageously, PEG having an average molecular weight of about400 is used in a method according to the invention.

The preferred average molecular range for PEG can be 300-500 g/molbecause PEG moieties having MW <300 lack sufficient hydrophilicity whilePEG moieties having MW >500 g/mol. impair permeability or stability.

An unsaturated fatty amine may be used in a method according to theinvention as a mild reducing and stabilizing agent or to produce a morehydrophobic SPION. Advantageously, oleylamine is used. An unsaturatedfatty amine such as oleylamine can function as a solvent for a reactionmixture, as coordinating agent to stabilize the surface ofnanoparticles, or as a coordinator with metal ions, and thus affect thekinetics of nanoparticle formation during their synthesis.

As alternatives to oleylamine, oleic acid and other unsaturated aminessuch as oleylamine acetate, oleylamine hydrofluoride may be utilized tosynthesized SPIONs having hydrophobic characteristics. NMR is oftenapplied to the human body in clinical applications. The same physicalprinciples involved in clinical imaging also apply to imaging anyfluid-saturated porous media, including reservoir rocks. Nuclearmagnetic resonance (NMR) may be used as a tool to interrogate ageological formation that may contain a liquid reservoir, such as a gas,oil or water reservoir. Typically an external magnetic field is imposedin the formation to make a measurement that is proportional to theporosity, regardless of lithology. This allows identification of thefree- and bound-fluid volumes and the free-fluid type (gas, oil orwater) and indication of permeability. In some embodiments of theinvention the SPIONs disclosed herein are used as contrast agents forNMR or MRI; seehttp://_www.halliburton.com/public/lp/contents/Books_and_Catalogs/web/NMR-Logging-Principles-and-Applications.pdf(last accessed Aug. 20, 2018, incorporated by reference).

In some embodiments of the invention, hydrophilic and/or hydrophobicSPIONS are injected during drilling of a borehole and permit NMRinterrogation along the length of the borehole. In contrast to MM wherea subject is placed at the center of an MRI instrument, geologicallogging places the instrument itself in a wellbore at a location withinthe geological formation to be analyzed by magnetic resonance imaginglogging or MRIL. In some embodiments, hydrophilic SPIONs areincorporated into a water-based mud and hydrophobic SPIONs into anoil-based mud during logging.

Generally, SPIONs are injected into the borehole as oil-based orwater-based colloidal dispersion itself. However, these SPIONs can be apart of drilling fluid and enhanced oil recovery (EOR) package.Commercially available NMR probes may be used to measure T₁ and T₂relaxation signals.

In other embodiments, the hydrophilic and/or hydrophobic SPIONs of theinvention may be contacted or incorporated into a rock sample obtainedfrom a geological formation which is then interrogated by NMR in alaboratory to assess porosity, permeability, water saturation, fluiddisplacement, hydrocarbon typing, etc.

Generally, cylindrical rock cores are selected for NMR measurements.Briefly, the cores are saturated with a brine solution followed bysaturation with crude oil or vice versa. The brine-oil saturated coresare completely scanned with NMR spectrometer. Then cores are cleaned byadopting a standard cleaning procedure using toluene. Similarly, thecores are saturated with a brine solution having hydrophilic SPIONsfollowed by saturation with crude oil. The saturated cores havingcontrast agents are scanned again with NMR spectrometer. Nuclearmagnetic resonance (NMR) logging is a type of well logging that uses theNMR response of a formation to directly determine its porosity andpermeability. It provides a continuous record along the length of aborehole. NMR logging measures the induced magnet moment of hydrogennuclei (protons) contained within the fluid-filled pore space of porousmedia (reservoir rocks). Unlike conventional logging measurements (e.g.,acoustic, density, neutron, and resistivity), which respond to both therock matrix and fluid properties and are strongly dependent onmineralogy, NMR-logging measurements respond to the presence of protons(e.g., in hydrogen). Because these protons primarily occur in porefluids, NMR effectively responds to the volume, composition, viscosity,and distribution of these fluids, for example, oil, gas or water.

An important mechanism affecting NMR relaxation is grain-surfacerelaxation. Molecules in fluids are in constant Brownian motion,diffusing about the pore space and bouncing off the grain surfaces. Uponinteraction with the grain surface, hydrogen protons can transfer somenuclear spin energy to the grain contributing to T1 relaxation orirreversibly dephase contributing to T2 relaxation. Therefore, the speedof relaxation most significantly depends on how often the hydrogennuclei collide with the grain surface and this is controlled by thesurface-to-volume ratio of the pore in which the nuclei are located.Collisions are less frequent in larger pores, resulting in a slowerdecay of the NMR signal amplitude and allowing a petrophysicist tounderstand the distribution of pore sizes.

NMR logs provide information about the quantities of fluids present, theproperties of these fluids, and the sizes of the pores containing thesefluids. From this information, it is possible to infer or estimate thevolume (porosity) and distribution (permeability) of the rock porespace, rock composition, type and quantity of fluid hydrocarbons, andhydrocarbon producibility. NMR logging provides measurements of avariety of critical rock and fluid properties in varying reservoirconditions (e.g., salinity, lithology, and texture), some of which areunavailable using conventional logging methods and without requiringradioactive sources. Whether run independently as a standalone serviceor integrated with conventional log and core data for advanced formationand fluid analyses, NMR logging has significantly contributed to theaccuracy of hydrocarbon-reservoir evaluation. Wireline-logging devicesare commercially available as are logging-while-drilling (LWD) devicesand downhole NMR spectrometers; seehttps://_petrowiki.org/Nuclear_magnetic_resonance_(NMR)_logging (lastaccessed Aug. 13, 2018, incorporated by reference).

NMR logging is typically performed using wireline tool orlogging-while-drilling (LWD) tools. In the conventional wireline-loggingtechnology, NMR logging is performed as the logging tool is beinglowered into a drilled borehole. In the emerging LWD technology, thelogging tools are generally rigged up as a part of the drilling stringand follow a drill bit during actual well drilling. Each tool type hasits own advantages. The wireline-tools enable high logging speeds andhigh-quality measurements. The LWD tools, on the other hand, providereal-time data during drilling operations that may be used to preventloss of circulation, blowouts, stuck pipes, hole instability and otherdisastrous consequences of borehole drilling.

The SPIONs disclosed herein may be suspended in water or sea water orother aqueous, non-aqueous, or emulsion compositions, such as drillingmuds and used as contrast agents.

Drilling muds are classified based on their fluid phase, alkalinity,dispersion and chemical components. They may be dispersed systems suchas freshwater muds that have a low pH (7.0-9.5) and may include spud,bentonite, natural or artificial polymers, phosphate treated muds,organic mud and organic colloid treated mud. Other freshwater mudsinclude high pH muds such as alkaline tannate-treated muds having a pHof 9.5 or more. Water based drilling muds can repress hydration anddispersion of clay and include high pH lime muds, low pH gypsum,seawater and saturated salt water muds. Water-based drilling mud mostcommonly contains or consists of bentonite clay (gel) with additivessuch as barium sulfate (barite), calcium carbonate (chalk) or hematite.Various thickeners are used to influence the viscosity of the fluid,e.g. xanthan gum, guar gum, glycol, carboxymethylcellulose, polyanioniccellulose (PAC), or starch. Defloculants can be used to reduce viscosityof clay-based muds; anionic polyelectrolytes (e.g. acrylates,polyphosphates, lignosulfonates (Lig) or tannic acid derivates such asQuebracho) are frequently used.

Non-dispersed system muds include low solids mud and emulsions. Lowsolids muds contain less than 3-6% solids by volume and weight less than9.5 lbs/gal. Most muds of this type are water-based with varyingquantities of bentonite and a polymer. Two types of emulsion muds areoil in water (oil emulsion muds) and water in oil (invert oil emulsionmuds). Oil based muds contain oil as the continuous phase and water as acontaminant, and not an element in the design of the mud. They typicallycontain less than 5% (by volume) water. Oil-based muds are usually amixture of diesel fuel and asphalt, however can be based on producedcrude oil and mud.

Examples

The following examples illustrate various aspects of the presentinvention. They are not to be construed to limit the claims in anymanner whatsoever.

As described in more detail below, the surfaces of SPIONs werefunctionalized by bonding polyethylene glycol (PEG-400) or oleylamine(OLA) on their surfaces to respectively provide hydrophilic andhydrophobic properties to incorporate into aqueous (e.g., water,seawater, brine) or oil (e.g., crude oil) materials in a geologicalreservoir. Uncoated SPIONs were also prepared by coprecipitation methodusing NH₄OH as a reducing agent for comparison. Stability of hydrophilicSPIONs was monitored in deionized (DI) water and/or artificial seawater(ASW), while stability of hydrophobic SPIONs was investigated in modeloil (cyclohexane-hexadecane 1:1).

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS)profiles confirmed the magnetite (Fe₃O₄) phase of synthesizednanoparticles (NPs). The presence of C—O (532.4 eV) and —NH₂ (399.7 eV)in XPS spectra of N1s and O1s substantiated the surfacefunctionalization of Fe₃O₄ NPs with PEG and OLA, respectively.Transmission electron microscopy (TEM) images demonstrated the sphericalshape NPs having particle diameters 11.6±1.4, 12.7±2.2 and 9.1±3.0 forPEG-Fe₃O₄, OLA-Fe₃O₄, and Fe₃O₄, respectively. NMR T₂-relaxationmeasurements were performed by an acorn area analyzer and demonstratedmeaningful results for NP (SPION) use in targeted reservoirapplications, for example, the transversal relaxivity (r₂) values forPEG-Fe₃O₄ (66.7 mM⁻¹s⁻¹) and OLA-Fe₃O₄ (49.0 mM⁻¹s⁻¹) were surprisinglyfound to be 2.07 and 1.53 times higher than those for Fe₃O₄ (32.2mM⁻¹s⁻¹) NPs, respectively.

Synthesis of hydrophilic SPIONs. The hydrophilic magnetite (Fe₃O₄) NPs(SPIONs) were prepared by a solvothermal method using PEG-400 (Aldrich).Briefly, 6 mmol (2.185 g) of iron(III) acetylacetonate Fe(acac)₃ (97%,Fluka) and 75 g of PEG-400 were mixed with the help of a Silverson mixer(L5M-A, USA) in an 125 mL polytetrafluoroethylene (PTFE) vessel for 1 hrto obtain a homogenous red suspension at room temperature. The PTFEvessel was placed in a stainless steel autoclave reactor (Parr, USA) andkept in a synthetic oven (280A, Fisher Scientific) at 180° C. for 24 h.Then, the mixture was cooled down to room temperature and the blackslurry of Fe₃O₄ was precipitated by the addition of absolute ethanol(>99%, Fisher Scientific) with an excess amount of diethyl ether (>99%,Sigma-Aldrich). The NPs were centrifuged at 10,000 rpm for 10 min using3-30KS centrifuge (Sigma, Germany). To remove unbound PEG-400, the NPswere redispersed in absolute ethanol and centrifuged again at 20,000 rpmfor 30 min. The purification procedure was repeated three-times. Thefinal black product was labeled as PEG-Fe₃O₄ and divided into two equalparts. Then, half of the product was dispersed in Milli-Q water whileremaining half was dried in vacuum oven at 50° C. for 24 h.

Synthesis of hydrophobic SPIONs. The hydrophobic Fe₃O₄ NPs weresynthesized by a solvothermal method using OLA (70%, Aldrich). Briefly,5 mmol (1.820 g) of Fe(acac)₃ precursor and 25 mL of OLA were mixed withthe help of a Silverson mixer in 125 mL PTFE vessel for 1 h to obtain ahomogenous red suspension. The PTFE vessel was placed in a stainlesssteel autoclave reactor and kept at 280° C. for 24 h. Then, the mixturewas cooled down to room temperature. The precipitation and purificationprocedure of the synthesized NPs remained same as described above forthe synthesis of hydrophilic Fe₃O₄. The final black product was labeledas OLA-Fe₃O₄ and divided into two equal parts. Then, half of the productwas dispersed in cyclohexane-hexadecane (1:1) mixture, while theremaining half was dried in vacuum oven at 50° C. for 24 h.

Synthesis of uncoated SPIONs. The uncoated-Fe₃O₄ NPs were prepared bycoprecipitation of Fe(III) and Fe(II) in the molar ratio (2:1) usingNH₄OH solution as a reducing agent. The complete reaction was carriedout under an Ar atmosphere and the stirring was carried out by usingoverhead Teflon stirrer (IKA Eurostar, Germany). In a typical procedure,100 mL of Milli-Q water was acidified with 1.0 mL of concentrated HCl(37%, Sigma-Aldrich) and purged with Ar gas for 15 min. Then, 1.2 MFeCl₃⋅6H₂O (>99%, Sigma-Aldrich) and 0.6 M FeCl₂⋅4H₂O (>99%,Sigma-Aldrich) aqueous solutions were prepared in acidified water. Thesolutions were filtered-off with 0.2-micron hydrophobic PTFE membranefilter (Millex-FG, Millipore). Then, Fe(II) solution was mixed dropwisewith Fe(III) solution in a three-neck round bottom flask. The reactionmixture was heated up to 80° C. and 20 mL of NH₄OH (28-30%,Sigma-Aldrich) solution was poured into the iron precursors at 500 rpm.The color of dispersion changed from golden brown to black indicatingthe formation of Fe₃O₄ NPs. The dispersion was continuously stirred,refluxed and heated for 1 hr followed by the addition of 5 mLtetramethylammonium hydroxide (25%, Sigma-Aldrich) solution to stabilizethe NPs. Then, the reaction mixture was allowed to cool down to roomtemperature. The magnetic NPs were washed several times with absoluteethanol as described above. The final product was labeled as Fe₃O₄ anddivided into two equal parts. Half of the product was dispersed inmilli-Q water while remaining half was vacuum dried in the oven at 50°C. for 24 h.

Functionality and colloidal stability test. The hydrophilicfunctionality and colloidal stability of as-synthesized SPIONs (i.e.,PEG-Fe₃O₄ and Fe₃O₄) were tested in deionized (DI, pH ˜7.0) water aswell as artificial seawater (ASW, pH ˜8.0). ASW was prepared which meetsAmerican standard for testing and materials (ASTM). Briefly, ASWaccording to the ASTM D1141-98 standard was prepared by dissolving 36.03g·L⁻¹ of a salt mixture in DI water. The composition of salt mixture wasas follows; NaCl (99.5%, 24.53 g), MgCl₂ (98%, 5.20 g), Na₂SO₄ (99%,4.09 g), CaCl₂ (99.9%, 1.16 g), KCl (99%, 0.695 g), NaHCO₃ (99.7%, 0.201g), KBr (99%, 0.101 g), H₃BO₃ (99.5%, 0.027 g), SrCl₂ (99.9%, 0.025 g)and NaF (99%, 0.003 g). The estimated density and salinity of ASW were1.020 g·mL⁻¹ and 36.0 gL⁻¹ respectively. However, the hydrophobicfunctionality and colloidal stability of as-synthesized OLA-Fe₃O₄ NPswas monitored in standard model oil composed of the mixture ofcyclohexane and hexadecane (1:1). For each test, as-synthesized SPIONswere dispersed in a bottle containing both, the model oil and ASW (1:1).Then, the functionality of NPs was investigated in terms of theirhydrophilic or hydrophobic characteristics, while the stability of NPswas monitored in their respective media.

Material characterization. The diffraction patterns of various SPIONswere recorded using a Smart Lab X-ray diffractometer (Rigaku, Japan)with a diffraction angle (2θ) range of 15-80° at a scan rate of 2°/min.Surface analysis of the synthesized magnetic materials was performedusing an X-ray photoelectron spectrometer (ESCALAB 250Xi, ThermoScientific, UK). The thermal behavior of functionalized NPs was studiedusing differential scanning calorimeter (DSC 204 F1 Phoenix, NETZSCH,Germany). DSC measurements were performed in the temperature range20-500° C. with a scan rate of 10° C./min under N₂ environment to avoidmaterial oxidation. The surface morphology, size, and shape of thesynthesized SPIONs were evaluated by using a field emission scanningelectron microscope (FESEM-Tescan Lyra-3) as well as a transmissionelectron microscope (JEM-2100, JEOL, USA). TEM grids were coated byputting slurry of the analyte onto 200 mesh copper grids. The grids wereexamined after 1 hr of initial degassing under vacuum. An inductivelycoupled plasma atomic emission spectrometer (ICP-AES, Varian) was usedto estimate the Fe content in as-synthesized SPIONs.

To determine the feasibility of contrast agents, T₂-relaxation curvesfor various concentrations of SPIONs were attained using an acorn areaanalyzer (Xigo Nanotools, UK), which is normally used for surface areameasurements; R. Tantra, Nanomaterial Characterization: An Introduction,John Wiley & Sons, 2016. The suitability of this miniaturized techniqueto obtain meaningful results is demonstrated here first time by showingits potential for targeted reservoir applications. For all themeasurements, values of tau (τ) and the total number of scans were keptconstant, i.e., τ=0.5 ms, scans=4.

Functionality and colloidal stability of synthesized SPIONs. Thefunctionality and colloidal stability of SPIONs are important factorsrelated to their ultimate use in oil exploration industries forreservoir applications. FIG. IA demonstrates functionality tests of (a)PEG-Fe₃O₄, (b) OLA-Fe₃O₄, and (c) Fe₃O₄ in an oil-DI water environment.The partitioning observed by naked eye showed that PEG-Fe₃O₄ and Fe₃O₄NPs had hydrophilic characteristics due to the presence of PEG and Oilsurface functional groups, respectively The ⁻presence of OH groups onthe surface of Fe₃O₄ NPs may be due to the hydroxylation process duringcoprecipitation; H. R. Shaterian and M. Aghakhanizadeh, Catal. Sci.Tech., 2013, 3, 425-428. Moreover, OLA-Fe₃O₄ NPs remained in theoil-phase owing to the presence of OLA functionality, which inducessurface hydrophobicity.

The colloidal stability of as-synthesized SPIONs was monitored in mixedoil-DI water and oil-seawater phases. FIG. 1A shows the stability of (a,d) PEG-Fe₃O₄, (b, e) OLA-Fe₃O₄, and (c, Fe₃O₄ in the oil-DI water phase.It was observed that PEG-Fe₃O₄ and OLA-Fe3O₄ NPs remained stable andattracted in an external magnetic field in their respectiveenvironments. However, the uncapped-Fe₃O₄ NPs became unstable and werenot fully attracted after applying the magnetic field. This may beattributed to the pH-dependence of Fe₃O₄ NPs which may have surfacesthat are easily oxidized to other forms of iron oxides/hydroxides in anaqueous media having pH ≤7; Sayar, 2006, id.

Similarly, FIG. 1B depicts the stability of (samples a, d) PEG-Fe₃O₄,samples b, e) OLA-Fe₃O₄, and (samples c, f) Fe₃O₄ in oil-seawaterphases, Similar behavior was observed for PEG-Fe₃O₄ and OLA-Fe₃O₄ NPsThe uncapped-Fe₃O₄ NIPs became stable and re attracted b: an appliedmagnetic field in ASW which has a slightly basic pH of about 8.0.

Crystal structure, phase, and chemical composition analysis. The phase,purity and crystal structures of as-synthesized SPIONs were examined viaXRD analysis. FIG. 2 shows the diffractograms of three types of magneticNPs synthesized using PEG, OLA, and NH₄OH. The observed diffractionprofiles are consistent with the standard pattern (JCPDS card no.65-3107) indicating the formation of pure magnetite phase. The six majordiffraction peaks observed at 2θ positionsof 30.08°, 35.50°, 43.22°,53.65°, 57.13° and 62.75° assigned to (2 2 0), (3 1 1), (4 0 0), (4 22), (5 1 1) and (4 4 0) crystalline planes, respectively; H. Sun, B.Chen, X. Jiao, Z. Jiang. Z. Qin and D. Chen, J. Phys. Chem. C. 2012,116, 5476-5481 and P. L. Hariam, M. Faizal and D. Setiabudidaya, IJESD,2013, 4, 336. According to structure analysis, the magnetite NPsexhibited the cubic inverse spinel structure with Fd-3m space group; R.M. Cornell and U. Schwertmann, The iron oxides: structure, properties,reaction occurrences and uses, John Wiley & Sons. 2003, The comparisonof XRD profiles revealed that peak intensities decreased after surfacefunctionalization of magnetite NPs due to the amorphous nature ofcapping agents (PEG-400 and OLA) which may indicate the coating ofSPIONs A. M. Atta, H. A. Al-Lohedan and S. A. Al-Hussain, Int. J. Mol.Sci., 2015, 16, 6911-6931. The average crystallite sizes of magnetiteNPs as evaluated using Debye-Scherrer equation (I. Khan, S. Ali, M.Mansha and A. Qurashi, Ultrason, Sonochem., 2017, 36, 386-392) werefound to be ˜13.3, ˜14.1 and ˜9.6 nm for PEG-Fe₃O₄, OLA-Fe₃O₄, andFe₃O₄, respectively. The cubic unit cell parameters (a) and cell volume(V) for as-synthesized NPs are reported in Table 1.

TABLE 1 Comparison of various parameters of as-synthesized SPIONs. UnitUnit Crystalline cell cell TEM Synthesis Reducing size parameter volumePhase diameter SPIONs method agent (nm) (Å) (Å³) composition (nm)PEG-Fe₃O₄ Solvothermal Polyethylene 13.3 8.377 587.9 Magnetite 11.6 ±1.4 180° C., 24 h glycol-400 OLA-Fe₃O₄ Solvothermal Oleylamine 14.18.385 589.4 Magnetite 12.7 ± 2.2 280° C., 24 h Fe₃O₄ CoprecipitationAmmonium 9.6 8.299 571.7 Magnetite  9.1 ± 3.0 80° C., 1 h hydroxide

The comparison indicates that PEG-Fe₃O₄ and OLA-Fe₃O₄ have almostsimilar values of unit cell parameters, perhaps owing to the samesynthetic protocol (solvothermal method). However, the uncapped Fe₃O₄NPs synthesized via the co-precipitation method possess lower unit cellparameters. This difference of values sing ests that synthetic protocolsplay a pivotal role in controlling the crystal structure of NPs; Y. V.Kolen'ko, M. Bañobre-López, C. Rodriguez-Abreu, E. Carbó-Argibay, ASailsman, Y. Piñeriro-Redondo, M. F. Cerqueira, D. Y. Petrovykh, Kovnirand O. I. Lebedev. J. Phys. Chem. C, 2014; 118, 8691-8701. It iswell-documented that magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) exhibitalmost similar XRD patterns; X, Zhang, Y. Niu, X. Meng, Y. Li and J.Zhao, CrystEngComm, 2013, 15, 8166-8172. Therefore, the phase analysisof as-synthesized SPIONs was further explored via XPS technique, whichexclusively determines various phases of iron oxides, i.e., magnetite,maghemite, and hematite.

A survey of PEG-Fe₃O₄, OLA-Fe₃O₄, and Fe₃O₄ spectra (FIG. 3A) shows thepresence of Fe2p, N1s, O1s, and C1s features. The observed spectrallines are labeled according to their binding energies. For C1s, theadventitious/aliphatic carbon (C—C) peak detected at 284.8 eV was usedas a reference. FIG. 3B shows XPS profiles of PEG-Fe₃O₄, OLA-Fe₃O₄, andFe₃O₄ freshly synthesized powder samples. The observed spectra arealmost similar to each other as well as with the standard Fe₃O₄ sample;T. Yamashita and P. Hayes, Appl. Surf Sci., 2008, 254, 2441-2449. It isreported elsewhere that Fe 2p _(3/2) does not have a satellite peak forFe₃O₄ phase; G. Wang, et al., 2014, id,. Liu, et al., 2015, id. and T.Yamshita, et al., 2008, id. Similarly, the absence of satellite peak at719.0 eV here further confirms the magnetite phase of as-synthesizediron oxide NPs. Moreover, the two distinct asymmetric peaks observed atbinding-energy values of 710.6 and 724.1 eV (spin-orbit splitting,Δ=13.5 eV) corresponded to Fe2p _(3/2) and Fe2p_(½) further confirmFe₃O₄ phase (711.0 and 724.6 eV in the case of γ-Fe₂O₃ phase); T.Yamshita, et al., 2008, id.

Surface functionalization. The presence of hydrophilic and hydrophobiccoating on the surface of SPIONs was investigated using twocomplementary techniques: XPS and DSC.

The XPS profiles also give evidence for the presence of an amorphouscoating on the surface of the NPs. The presence of amine (—NH₂) groupsin XPS spectrum of OLA-Fe₃O₄ indicates surface functionalization of theNPs. It is reported that the binding-energy values corresponding tobonded amines are observed in the range 398-400 eV; M. Aslam, E. A.Schultz, T. Sun, T. Meade and V. P. Dravid, Cryst. Growth Des., 2007, 7,471-475. A symmetric peak with low intensity detected at 399.7 eV in theNls spectrum (FIG. 3C) attributed to ^(—)NH₂ group of OLA in whichnitrogen is coordinated with metal oxide NPs; W. Lu, M. Ling, M. Jia, P.Huang, C. Li and B. Yan, Mol. Med. Rep., 2014, 9, 1080-1084. Theobserved binding-energy is consistent with the literature value foramine-capped NP; M. Aslam, E. A. Schultz, T. Sun, T. Meade and V. P.Dravid, Cryst. Growth Des., 2007, 7, 471-475and D. Wilson and M.Langell, Appl. Surf Sci., 2014, 303, 6-13. FIGS. 3D-3F reveal thedeconvoluted high-resolution XPS spectra of the O1s component forPEG-Fe₃O₄, OLA-Fe₃O₄, and Fe₃O₄, respectively. Two characteristic peaksobserved in all O1s spectra at ˜530.0 and ˜531.6 eV belong to Fe—O ofiron oxide NPs core (Y. V. Kolen'ko, et al., 2014, id) and hydroxylgroup (—OH)), respectively. Additionally, a strong shoulder peakdetected at 532.4 eV in O1s profile of PEG-Fe₃O₄ (FIG. 3D) assigned toC—O group of PEG-400 in which oxygen atoms are bonded to aliphaticcarbon; D. Maity, et al., 2009, id. and B. Gupta, N. Kumar, K. Panda, A.A. Melvin, S. Joshi, S. Dash and A. K. Tyagi, J. Phys. Chem. C, 2016,120, 2139-2148. The existence of Fe—O and C—O confirms the surfacefunctionalization of Fe₃O₄ NPs with PEG groups.

DSC was further employed to investigate the organic surface coating,thermal stability and phase transformations of the magnetite NPs atelevated temperature. FIG. 4 shows the thermal behavior of uncoated andcoated SPIONs in the temperature range of 20-500° C. From the DSC curveof uncoated-Fe₃O₄, it is quite clear that the NPs are almost thermallystable and no phase transformation is observed up to 500° C. under an N₂environment. However, a small exothermic process observed in thetemperature range 125-200° C. which could be attributed to the removalof hydroxyl (—OH) groups adsorbed during coprecipitation on the surfaceof uncoated-Fe₃O₄. In the case of coated NPs (PEG-Fe₃O₄ and OLA-Fe₃O₄),DSC curves show multi-step exothermic processes in the range of 150-400°C. with maxima at 279 and 275° C., which are attributed to thedecomposition of organic capping agents, i.e., PEG-400 and OLAfractions, respectively; A. Mukhopadhyay, N. Joshi, K. Chattopadhyay andG. De, ACS Appl. Mater. Interfaces, 2011, 4, 142-149 and A. Monfared, A.Zamanian, M. Beygzadeh, I. Sharifi and M. Mozafari, J. Alloys Compd.,2017, 693, 1090-1095. The observed thermal behavior of PEG-Fe₃O₄ andOLA-Fe₃O₄ confirmed the functionalization of SPIONs.

Surface morphology and particle size analysis. Surface morphology andparticle size of as-synthesized SPIONs were investigated via FESEM andTEM techniques. High and low-resolution FESEM images of (a) PEG-Fe₃O₄,(b) OLA-Fe₃O₄, and (c) Fe₃O₄ are shown in FIGS. 5 and 8 , respectively.From the micrographs of PEG-Fe₃O₄ and OLA-Fe₃O₄, it can be clearlyobserved that the synthesized SPIONs have almost a spherical shape andsingle distribution. However, aggregation and lumps have been observedin the case of uncapped-Fe₃O₄ NPs synthesized by the coprecipitationmethod, as indicated by red circles in FIG. 5C.

The comparison indicates that the solvothermal protocol allows controlof shape and size of NPs as compared to the coprecipitation method.Spherical shaped NPs are predominantly formed in the synthesis of Fe₃O₄owing to the low surface area per unit volume, and hence minimum surfacefree-energy; D. K. Kim, M. Mikhaylova, Y. Zhang and M. Muhammed, Chem.Mater., 2003, 15, 1617-1627. This is attributed the nucleation rate perunit area which is isotropic at the NP interfaces, which results inminimization of surface free-energy; D K. Kim, et al., 2003, id.Therefore, the equivalent growth rate in all directions of nucleationleads to the formation of high and low resolution TEM images of (a)PEG-Fe₃O₄, (b) OLA-Fe₃O₄, and (c) Fe₃O₄ are shown in FIGS. 6A-6C, FIGS.8A-8C, and FIGS. 9A-9C, respectively. The images clearly indicate thatPEG-Fe₃O₄ and OLA-Fe₃O₄ NPs exhibited spherical morphology andhomogenous distributions, whereas uncoated-Fe₃O₄ NPs exhibit irregularshapes. The average particle diameter of as-synthesized NPs is 11.6±1.4,12.7±2.2 and 9.1±3.0 for PEG-Fe₃O₄, OLA-Fe₃O₄, and Fe₃O₄, respectively(Table 1). The broad distribution observed for uncapped-Fe₃O₄ NPsindicates that the particle size was not well-controlled with thecoprecipitation method. The comparison also indicates that particlediameters estimated from TEM images are consistent with averagecrystallite sizes observed from XRD. The selected area electrondiffraction (SAED) patterns of (d) PEG-Fe₃O₄, (e) OLA-Fe₃O₄, and (f)Fe₃O₄ SPIONs (FIG. 6A-6F) are consistent with XRD profiles (FIG. 2 ) andliterature; A. Mukhopadhyay, et al., 2011, id. The SAED patterns alsoexhibit structural homogeneity and a high degree of crystallinity of thesynthesized NPs. The patterns are indexed based on cubic inverse spinelstructure with Fd-3m space group and unit cell parameters of magnetiteNPs (Table 1).

Growth mechanism of SPIONs. The possible growth mechanism of theseas-synthesized SPIONs is proposed below. The PEG-400 and OLA can beconsidered high-boiling solvents playing three roles (reducing,stabilizing, and capping agents) in the solvothermal synthesis ofSPIONs. The mechanism of Fe₃O₄ NPs formation may become more complicatedwhen metal-organic salts Fe(acac)₃ are used as precursors. At anelevated temperature, Fe(acac)₃ precursor decomposes and liberate Fe³⁺ions. PEG-400 and OLA are oxidized at high temperature and generateelectrons reducing Fe³⁺ to Fe²⁺. PEG-400 is a stronger reducing agentand generates Fe₃O₄ NPs at a relatively low temperature (e.g., 180° C.),whereas OLA, being a mild reducing agent generates the NPs at arelatively higher temperature (e.g., 280° C.). These organicsolvent/additives effectively controlled the particle growth andprevented aggregation. Spherical-shaped NPs were predominantly formeddue to the minimum surface free-energy. However, the synthesis of SPIONsby the coprecipitation method using Fe³⁺ and Fe²⁺ ions was pH-dependentbased on the following chemical reaction (F. Sayar, G. liven and E.Pişkin, Colloid Polym. Sci., 2006, 284, 965):

2Fe³⁺+Fe²⁺+8 OH⁻→Fe₃O₄+4 H₂O  (1)

According to above equation (1), a complete co-precipitation of Fe₃O₄NPs was observed for pH above 7, while also keeping the molar ratio(2:1) between Fe³⁺ and Fe²⁺ under a non-oxidizing environment. In thiscase, pH was adjusted to ˜9.0 using NH₄OH as a precipitating agent andthe NPs were stabilized with tetramethylammonium hydroxide solution.

T₂-relaxation and relaxometric studies. Spin-spin relaxation NMR(T₂-relaxation) measurements were performed to investigate thepossibility employing these SPIONs as T₂-contrast agents for oilreservoir applications. The measurements were carried out for variousconcentrations of Fe in the as-synthesized SPIONs as shown in FIGS.7A-7D.

Before T₂-measurements, the Fe contents present in the samples wereestimated with the help of ICP-AES analysis and were determined to be57.9, 61.5 and 68.8 wt % of Fe content for PEG-Fe₃O₄, OLA-Fe₃O₄, andFe₃O₄, respectively. Six concentrations of Fe (mM), i.e., 0.012, 0.024,0.060, 0.12, 0.24 and 0.48 were prepared to determine the relaxometricproperties of hydrophilic and hydrophobic samples in ASW and model oil,respectively.

T₂-relaxation measurements of (a) PEG-Fe₃O₄, (b) OLA-Fe₃O₄, and (c)Fe₃O₄ SPIONs with respect to Fe concentration are shown in FIGS. 7A-7D.A significant quenching of T₂-relaxation signals was observed withincreasing concentrations of SPIONs. For comparison, T₂-relaxationcurves and relaxation times of pure DI water, ASW, cyclohexane,hexadecane, and model oil are shown by FIG. 10 .

The relaxation process took place due to energy exchange betweenneighboring protons in solvent molecules. SPIONs induced inhomogeneityin the presence of an applied magnetic field, which resulted in thede-phasing of magnetic moments of protons and led to the quenching ofthe T₂ signal. This decrease in T₂-relaxation time with Fe concentrationindicates that these NPs can act as T₂-contrast agents for oil reservoirapplications.

The relaxivity properties were investigated by plotting various Feconcentration (mM) against relaxation time (1/T₂, s⁻¹), as shown in FIG.7D. The r₂ value can be estimated from the slope of equation (2); N.Arsalani, H. Fattahi and M. Nazarpoor, Express Polym Lett, 2010, 4,329-338.

1/T ₂=1/T ₂ °+r ₂[Fe]  (2)

Where, T₂, T₂°, R₂, and [Fe] are the relaxation time of NPs dispersion,pure solvent, transversal relaxivity and iron concentration (mM), Theestimated r₂ values were found to be 66.7, 49.0, and 32.2 mM⁻¹ s⁻¹ forPEG-Fe₃O₄, OLA-Fe₃O₄, and Fe₃O₄SPIONs respectively.

The higher r₂ values for PEG-Fe₃O₄ and OLA-Fe₃O₄ indicated that thecapped-Fe₃O₄ showed excellent relaxivity properties owing to theirhigher dispersion in the respective media as compared to uncapped Fe₃O₄.

The estimated value for PEG-Fe₃O₄ was competitive with the commercialcontrasting agents such as SHU-555C (r₂=69 mM⁻¹ s⁻¹) and 10 times higherthan Gd-DTPA (r₂=5.3 mM⁻¹ s⁻¹)¹⁹. A comparison of various T₂- contrastagents is provided in Table 2.

TABLE 2 Comparison of various T₂-contrast agents for MRI applications.Particle Field Sample Synthesis Colloidal size strength r₂ compositionmethod stability (nm) (T) (mMs⁻¹) Refs. Fe₃O₄ Polyol H₂O, PBS 8 1.5 82.7A USMIO-Fe₃O₄ Coprecipitation H₂O 6.6 0.47 33.9 B MION-Fe₃O₄Coprecipitation — 4.6 — 34.8 C USPIO-Fe₃O₄ Coprecipitation 0.9% saline4.9 0.47 53.1 D US-Fe₃O₄ Coprecipitation pH: 5.3-8.5 4.6 7 64.4 EUS-Fe₃O₄ Coprecipitation pH: 5.3-8.5 2.2 7 28.6 E PEG-Fe₃O₄ SolvothermalH₂O, seawater 11.6 1.5 66.7 Inv. OLA-Fe₃O₄ Solvothermal H₂O, seawater12.7 1.5 49.0 Inv. US: Ultra-small, MIO: Magnetic iron oxide, PIO:Paramagnetic iron oxide, PBS: Phosphate buffered saline. Inv =invention. A = J. Wan, W. Cai, X. Meng and E. Liu, Chem. Commun.(Cambridge, U. K.), 2007, 5004-5006. B = E. V. Groman, J. C. Bouchard,C. P. Reinhardt and D. E. Vaccaro, Bioconjugate Chem., 2007, 18,1763-1771. C = T. Shen, R. Weissleder, M. Papisov, A. Bogdanov and T. J.Brady, Magn. Reson. Med., 1993, 29, 599-604. D = H. K. Pannu, K. P.Wang, T. L. Borman and D. A. Bluemke, J. Magn. Reson. Imaging, 2000, 12,899-904. E = G. Wang, X. Zhang, A. Skallberg, Y. Liu, Z. Hu, X. Mei andK. Uvdal, Nanoscale, 2014, 6, 2953-2963.

These outcomes suggest that theses functionalized SPIONs can beeffectively used as T₂-contrast agents for reservoir applications due totheir excellent relaxivity properties.

As shown herein, highly-stable hydrophilic and hydrophobic SPIONscontrast agents were successfully prepared using a single-stepsolvothermal method. Hydrophilic and hydrophobic characteristics wereinduced on the surfaces of the magnetite NPs by adsorbing either PEG-400or OLA, respectively. The additives (PEG-400 and OLA) played threeroles, i.e., as reducing, stabilizing, and capping agents during thesynthesis processes.

The hydrophilic and hydrophobic SPIONs were found to be stable in ASW(36.03 gL⁻¹ salt in distilled water) and model oil(cyclohexane-hexadecane 1:1), respectively, which is a requirement forefficient use in a harsh oil reservoir environment.

The magnetite phase having cubic inverse spinel structure with Fd-3mspace group was confirmed by XRD.

The surface functionalization of capped-NPs was established by thepresence of C—O and —NH₂ groups in XPS spectra. TEM images demonstratedthe spherical shape of as-synthesized NPs having ultra-small diameters<15 nm, which is a suitable size for passing through reservoir rockcores.

The suitability of NMR T₂-relaxation as a measurement tool, i.e.,miniaturized acorn area analyzer in this case was successfullydemonstrated here for the first time for oil reservoir applications.

The estimated r₂ value for PEG-Fe₃O₄ (66.7 mM⁻¹ s⁻¹) is competitive withthe commercial contrasting agents such as SHU-555C (r₂=69 mM⁻¹ s⁻¹) andhigher than Gd-DTPA (r₂=5.3 mM⁻¹ s⁻¹) (Z. Li, et al., 2012, id.) as wellas reported values in literature (Table 2).

The observed excellent relaxivity properties due to their ultra-smallsizes and long-term stability in the respective medium show thesehydrophilic and hydrophobic SPIONs to be useful T₂-contrast agents foroil reservoir applications. Moreover, these properties are consistentwith their utility as contrast agents for MRI and nanosensors for remoteinterrogation in both biomedical and oil reservoir applications.

Terminology. Terminology used herein is for the purpose of describingparticular embodiments only and is not intended to be limiting of theinvention.

The headings (such as “Background” and “Summary”) and sub-headings usedherein are intended only for general organization of topics within thepresent invention, and are not intended to limit the disclosure of thepresent invention or any aspect thereof. In particular, subject matterdisclosed in the “Background” may include novel technology and may notconstitute a recitation of prior art. Subject matter disclosed in the“Summary” is not an exhaustive or complete disclosure of the entirescope of the technology or any embodiments thereof. Classification ordiscussion of a material within a section of this specification ashaving a particular utility is made for convenience, and no inferenceshould be drawn that the material must necessarily or solely function inaccordance with its classification herein when it is used in any givencomposition.

As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise.

It will be further understood that the terms “comprises” and/or“comprising,” when used in this specification, specify the presence ofstated features, steps, operations, elements, and/or components, but donot preclude the presence or addition of one or more other features,steps, operations, elements, components, and/or groups thereof.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items and may be abbreviated as“/”.

Links are disabled by deletion of http: or by insertion of a space orunderlined space before www. In some instances, the text available viathe link on the “last accessed” date may be incorporated by reference.

As used herein in the specification and claims, including as used in theexamples and unless otherwise expressly specified, all numbers may beread as if prefaced by the word “substantially”, “about” or“approximately,” even if the term does not expressly appear. The phrase“about” or “approximately” may be used when describing magnitude and/orposition to indicate that the value and/or position described is withina reasonable expected range of values and/or positions. For example, anumeric value may have a value that is +/−0.1% of the stated value (orrange of values), +/−1% of the stated value (or range of values), +/−2%of the stated value (or range of values), +/−5% of the stated value (orrange of values), +/−10% of the stated value (or range of values),+/−15% of the stated value (or range of values), +/−20% of the statedvalue (or range of values), etc. Any numerical range recited herein isintended to include all sub-ranges subsumed therein.

Disclosure of values and ranges of values for specific parameters (suchas temperatures, molecular weights, weight percentages, etc.) are notexclusive of other values and ranges of values useful herein. It isenvisioned that two or more specific exemplified values for a givenparameter may define endpoints for a range of values that may be claimedfor the parameter. For example, if Parameter X is exemplified herein tohave value A and also exemplified to have value Z, it is envisioned thatparameter X may have a range of values from about A to about Z.Similarly, it is envisioned that disclosure of two or more ranges ofvalues for a parameter (whether such ranges are nested, overlapping ordistinct) subsume all possible combination of ranges for the value thatmight be claimed using endpoints of the disclosed ranges. For example,if parameter X is exemplified herein to have values in the range of 1-10it also describes subranges for Parameter X including 1-9, 1-8, 1-7,2-9, 2-8, 2-7, 3-9, 3-8, 3-7, 2-8, 3-7, 4-6, or 7-10, 8-10 or 9-10 asmere examples. A range encompasses its endpoints as well as valuesinside of an endpoint, for example, the range 0-5 includes 0, >0, 1, 2,3, 4, <5 and 5.

As used herein, the words “preferred” and “preferably” refer toembodiments of the technology that afford certain benefits, undercertain circumstances. However, other embodiments may also be preferred,under the same or other circumstances. Furthermore, the recitation ofone or more preferred embodiments does not imply that other embodimentsare not useful, and is not intended to exclude other embodiments fromthe scope of the technology. As referred to herein, all compositionalpercentages are by weight of the total composition, unless otherwisespecified. As used herein, the word “include,” and its variants, isintended to be non-limiting, such that recitation of items in a list isnot to the exclusion of other like items that may also be useful in thematerials, compositions, devices, and methods of this technology.Similarly, the terms “can” and “may” and their variants are intended tobe non-limiting, such that recitation that an embodiment can or maycomprise certain elements or features does not exclude other embodimentsof the present invention that do not contain those elements or features.

Although the terms “first” and “second” may be used herein to describevarious features/elements (including steps), these features/elementsshould not be limited by these terms, unless the context indicatesotherwise. These terms may be used to distinguish one feature/elementfrom another feature/element. Thus, a first feature/element discussedbelow could be termed a second feature/element, and similarly, a secondfeature/element discussed below could be termed a first feature/elementwithout departing from the teachings of the present invention.

The description and specific examples, while indicating embodiments ofthe technology, are intended for purposes of illustration only and arenot intended to limit the scope of the technology. Moreover, recitationof multiple embodiments having stated features is not intended toexclude other embodiments having additional features, or otherembodiments incorporating different combinations of the stated features.Specific examples are provided for illustrative purposes of how to makeand use the compositions and methods of this technology and, unlessexplicitly stated otherwise, are not intended to be a representationthat given embodiments of this technology have, or have not, been madeor tested.

All publications and patent applications mentioned in this specificationare herein incorporated by reference in their entirety to the sameextent as if each individual publication or patent application wasspecifically and individually indicated to be incorporated by reference,especially referenced is disclosure appearing in the same sentence,paragraph, page or section of the specification in which theincorporation by reference appears.

The citation of references herein does not constitute an admission thatthose references are prior art or have any relevance to thepatentability of the technology disclosed herein. Any discussion of thecontent of references cited is intended merely to provide a generalsummary of assertions made by the authors of the references, and doesnot constitute an admission as to the accuracy of the content of suchreferences.

1-13. (canceled)
 14. A method for NMR logging of a geological formation,comprising injecting or otherwise incorporating hydrophilicsuperparamagnetic iron oxide nanoparticles (hydrophilic SPIONs) into thegeological formation, generating a magnetic field and detecting amagnetic signal, wherein the hydrophilic SPIONS are made by a processcomprising: mixing an iron-containing precursor with a hydrophilicligand or capping agent to form a homogeneous suspension; heating thehomogenous suspension to a temperature sufficient to bind thehydrophilic ligand to the iron-containing precursor, cooling theresulting homogenous suspension, adding a solvent to the cooledhomogeneous suspension to precipitate hydrophilic-Fe₃O₄ SPIONs into aslurry, separating the precipitated hydrophilic-Fe₃O₄ SPIONs from theslurry, and washing the hydrophilic-Fe₃O₄ SPIONs to remove unboundhydrophilic ligands or capping agent, thereby making the hydrophilicSPIONs, wherein the iron-containing precursor is iron(III)acetylacetonate, the hydrophilic ligand or capping agent is polyethyleneglycol 400 (PEG-400), the heating is at a pressure of about 15 to about45 psi and at a temperature ranging from 175to 185° C. for at least 12hours, the solvent is a mixture of absolute ethanol and diethylether,and the hydrophilic-Fe₃O₄SPIONs are washed in ethanol.
 15. The method ofclaim 14, wherein the magnetic signal is a T1 signal and/or a T2 signal.16-17. (canceled)
 18. The method of claim 14, wherein the hydrophilicSPIONs are injected or introduced into the geological formation througha borehole.
 19. The method of claim 14, wherein the hydrophilic SPIONsare injected into oil and/or water present a fluid reservoir in thegeological formation.
 20. The method of claim 14 that compriseslogging-while-drilling (LWD) logging or, wireline logging.
 21. A methodfor NMR logging of a geological formation, comprising injecting orotherwise incorporating hydrophobic superparamagnetic iron oxidenanoparticles (hydrophobic SPIONs) into the geological formation,generating a magnetic field and detecting a magnetic signal, wherein thehydrophobic SPIONs are made by a process comprising: mixing aniron-containing precursor with a hydrophobic ligand or capping agent toform a homogeneous suspension; heating the homogenous suspension to atemperature sufficient to bind the hydrophobic ligand or capping agentwith the iron-containing precursor, cooling the homogenous suspension,adding a solvent to precipitate hydrophobic-Fe₃O₄ SPIONs into a slurry,separating hydrophobic-Fe₃O₄ SPIONs from the slurry, and washing thehydrophobic-Fe₃O₄ SPIONs to remove unbound hydrophobic ligand or cappingagent, thereby making the hydrophobic SPIONs, wherein theiron-containing precursor is iron(III) acetylacetonate, the hydrophobicligand or capping agent is oleylamine (OLA), the heating is at apressure of 40 to 80 psi and at temperature ranging from 275 to 285° C.for at least 12 hours, the solvent is a mixture of absolute ethanol anddiethylether, and the hydrophobic-Fe₃O₄ SPIONs are washed in ethanol.22. The method of claim 21, wherein the magnetic signal is a T1 signaland/or a T2 signal.
 23. The method of claim 21, wherein the hydrophobicSPIONs are injected or introduced into the geological formation througha borehole.
 24. The method of claim 21, wherein the hydrophobic SPIONsare injected into oil and/or water present a fluid reservoir in thegeological formation.
 25. The method of claim 21 that compriseslogging-while-drilling (LWD) logging or wireline logging.